yeah its gotta be bugged it happened to me today and yesterday. then some asshat told me " it happens to all classes of all specs even before 5.1 came out". he obviously had no clue wtf he was saying. but yeah this should get hot fixed asap
That buff has to be the buggiest buff ever implemented into the game.
In 5.0.4 (which was when I was lvling my monk, so idk about 5.0.5) you could refresh a 2 hour 50% exp buff by simply doing one exp quest. And like Codyak said, as long as you didn't hit a loading screen the buff would simply stay on you forever.
It doesn't stack with the 8th anniversary buff, if that's on no monk buff for you (from what I've heard). Also it seems to only be giving +1 hour instead of bumping it up to a full 2 hours if you still have the buff when you turn in another quest. Someone on wowhead said there doesn't seem to be a cap to it anymore either.
My experience was that as long as you didn't hit loading screen, it lasted longer than the 1hr duration it showed. Since 5.1 however, I've had it last both the full duration of 1hr, and instantly disappear as I teleported back after completing the daily. Don't know if it has issues with the Anniversary buff, but I've had both on while leveling prior to 5.1 without any problems.